The bibulous Irish playwright Brendan Behan, banned from entering Britain for a solo IRA wartime bombing mission, was monitored by MI5 for several decades.

More than 45 years after his death from excessive drinking and burial with an IRA military salute over his grave, the security service's files, finally released, include a remark from 1956 that one "source considers that as an individual he is too unstable and too drunken to be particularly dangerous".

He had tried to blow up Liverpool Docks during an unauthorised raid, was caught and sent to prison for offences under the Explosive Substances Act.

In October 1941 he was deported back to Ireland and a notice circulated to passport offices that he should be refused entry. Back in Dublin, Behan was soon in trouble. After shooting at several Gardai officers, following a republican commemoration, he was arrested and sent to the city's Mountjoy Prison.

Correspondence with his stepbrother, Sean Furlong, who worked at the Royal Ordnance factory in Sellafield, was monitored by the intelligence services. "It's the futility of it all that's getting me down," Behan admitted from his cell in Mountjoy.

"Personally I think the Irish people are just about browned off with all this bloody game of private armies ... Sean, I am firmly convinced that republicanism (God almighty it's not even republicanism with the half of them) of this particular brand is defunct....